The present invention relates to a position adjusting method for a deflecting yoke of a color cathode-ray tube applied when the yoke is mounted on the tube, and especially to a position adjusting method comprising a special positioning of a color cathode-ray tube with respect to measuring equipment.
A color cathode-ray tube has, on the inner surface of the panel of the tube, a stripe-shaped or dot-shaped fluorescent surface having colors of green, blue and red arranged in accordance with the slots formed in a shadow mask located in the tube. In mounting a deflecting yoke on the tube, the deflecting yoke is first adjusted in its position relative to the fluorescent surface, and then secured to the tube. The position adjusting process for the yoke includes a luster position adjustment, a purity adjustment and a convergence adjustment. The luster position adjustment is achieved by rotating the yoke around the deflection axis, the purity adjustment is achieved by shifting the yoke relative to the fluorescent surface in the direction of the central axis of deflection, and the convergence adjustment is achieved by shifting the yoke in a plane perpendicular to the central axis of deflection, i.e. parallel to the fluorescent surface of the tube, namely in a plane including X and Y directions as indicated in the drawings. These adjustments run into several dimensions, and require much labor.
In the prior art, a cathode-ray tube is positioned in a state where the panel surface thereof faces in horizontal direction. Then, measuring equipment is attached to the cathode-ray tube, and the position of the yoke is adjusted. This method has problems as described below.
First of the, the adjustment is influenced by earth magnetism. Heretofore, in carrying out a purity adjustment, when an adjustment achieved in the case where the tube panel faces east is selected as a standard adjustment, this standard adjustment is to be confirmed in other situations such as when the panel faces north or south. In this adjusting method, however, the electron beam through the tube is influenced by the vertical magnetic field of the earth, and the positional relation between the electron beam and the fluorescent surface is disturbed.
Secondly, the dielectric strength of the tube against a high voltage becomes a problem. In a color cathode-ray tube having an electron gun of the multi-stage type where a high voltage is applied to each electrode, there is a risk that foreign matter which might exist in the tube may drop into the electron gun when the panel is directed horizontally for an adjustment, thereby deteriorating, due to the foreign matter, the dielectric strength against a high voltage which is applied to the electron gun.
Thirdly, there is a problem with respect to handling of the measuring equipment. In case where the panel surface faces in a horizontal direction, the measuring equipment is required to be pressed against the panel surface where the equipment is to be attached to the panel.